In recent years, it has been desired to develop an optical recording medium on which information can be recorded at a higher density as compared with a conventional one in order to record and retrieve a large quantity of data such as a long and high quality animation. Such an optical recording medium on which information can be recorded at a high density may, for example, be a laminated multilayer optical recording medium such as DVD-ROM having a laminated structure wherein two recording layers (dual layer) are formed on one medium. Employing such multilayer technology, it is possible to increase the capacity without changing the recording density per layer.
Such a laminated multilayer optical recording medium is produced usually by a production process called photopolymerization process (hereinafter sometimes referred to as “2P process”). By the 2P process, a two-layer structure optical recording medium is produced, for example, by forming on a transparent first substrate having a concavo-convex shape for record track formed thereon, a first recording layer, a first reflective layer, an interlayer having a concavo-convex shape for record track formed thereon, a second recording layer and a second reflective layer in this order, and finally bonding a second substrate.
In the 2P process, the interlayer is produced usually as follows. First, a light-curable resin material or the like curable by light (the light may, for example, be radiation ray such as ultraviolet ray) is applied to the first reflective layer to form a resin material layer, and then a stamper having a concavo-convex shape to be transferred (hereinafter optionally referred to as “concavo-convex shape for transfer”) is disposed thereon. Then, the light-curable resin material or the like is cured, and then the stamper is separated. In such a manner, the concavo-convex shape for transfer of the stamper is transferred on the surface of the light-curable resin to form the interlayer having a concavo-convex shape by the cured product of the curable resin.
Accordingly, in the 2P process, it is required that the stamper after the light-curable resin is cured is smoothly separated. Namely, if problems in production arise in formation of the interlayer having a concavo-convex shape for record track by the 2P process, such that the stamper is hardly separated from the light-curable resin, or that uniformity on the surface of the interlayer decreases even if the stamper is separated therefrom, it is likely that the interlayer will have defects such as scratches or peeling, and optical information can not stably be recorded on and retrieved from the optical recording medium.
Especially in a case where a stamper made of a polycarbonate resin or an acrylic resin is used, it is difficult to separate the stamper from an interlayer formed of e.g. an ultraviolet-curable resin (Patent Documents 1 and 2).
Patent Document 2 proposes to apply a surface coating by an inorganic material to a stamper made of an acrylic resin. And it is disclosed that the above-mentioned separation can thereby be carried out satisfactorily. Further, in the same document, an acrylic stamper is employed wherein a SiO2 dielectric film is formed on the surface of a groove/pit (corresponding to a concavo-convex shape for transfer) of the stamper.
On the other hand, Patent Document 3 proposes that the entire stamper is made of a cyclic polyolefin or polystyrene resin, or at least the surface of the stamper having a matrix pattern (corresponding to a concavo-convex shape for transfer) formed, is made of a cyclic polyolefin or polystyrene resin. And, it is disclosed that it is thereby possible to improve the releasability of the stamper from the interlayer made of a cured product of a radiation-curable resin. Further, Patent Document 3 indicates that a cyclic polyolefin makes the releasability of the stamper particularly good from the interlayer made of a cured product of a radiation-curable resin.    Patent Document 1: WO2005/048253 (paragraph [0100])    Patent Document 2: JP-A-2002-279707 (paragraphs [0021], [0028])    Patent Document 3: JP-A-2003-85839 (paragraphs [0006], [0016], [0046] to [0055], etc.)